In the field of bipolar and integrated circuits, particularly those circuits used in transistor-transistor logic (TTL) technology, bipolar junction transistors are sometimes "clamped" so as to restrict the forward biasing of the collector-base junctions. This is to prevent excess minority carrier storage in the base region while the transistor is "on," i.e., to prevent the transistor's going into "deep saturation" while conducting, a condition which severely limits the speed with which the transistor can be switched off. Deep saturation is particularly undesirable in the "pull-up" and "pull-down" transistors used in binary logic circuits where switching speed is essential.
For many applications, the voltage-clamping device of choice for this task has been the Schottky diode, which has a sharp knee at a low voltage in the forward i-v curve and also a reasonably low reverse saturation current. The collector-base junction is shunted by a Schottky diode. The clamping is effected by shunting the transistor's collector-base junction with a Schottky diode oriented so as to be forward-biased when the collector-base junction is forward-biased.
Modern Schottky diodes are formed by laying down a metallic film on a semiconductor. Provided the semiconductor carrier concentration is not too high, a rectifying junction--the Schottky junction--is at the metal/semiconductor interface; its key characteristics--Schottky barrier and reverse saturation current--are determined by the semiconductor carrier concentration at that interface. (If the concentration is too high, the interface will be ohmic rather than rectifying; there will not be a Schottky diode established.)
Prior art integrated circuits incorporating Schottky diodes are typically formed by depositing metal on the surface of an epitaxial layer of semiconductor material. See, for example, U.S. Pat. No. 4,849,344 issued to Desbiens et al. in 1989 and U.S. Pat. No. 4,943,742 issued to Fukushima in 1990. In many cases this same epitaxial layer forms a part of the collector region of the bipolar transistor. The base region is of opposite type semiconductor from that of the epitaxial layer (P type if the epitaxial layer is N type, and conversely). The transistor structure also includes an emitter region of the same conduction type as the collector region. Fabrication procedures for typical prior art transistor structures have been described in U.S. Pat. No. 3,648,125 issued to Peltzer in 1972, and U.S. Pat. No. 4,498,227 issued to Howell et al. in 1985. The monocrystalline epitaxial layer is typically doped uniformly with donors or acceptors and formed by vapor deposition on a semiconductor substrate of opposite conductivity type: i.e., if the substrate is N type material, the epitaxial layer will be N type, and conversely. (The substrate is usually of the same conductivity type as the base region of the transistor to be formed, though with a lower carrier concentration.) The epitaxial layers produced in most prior-art fabrication processes are doped essentially uniformly, with a typical surface concentration of about 1-3.times.10.sup.16 donors/cc, leading to a sheet resistance of about 3500-4500 ohms/sq and a bulk resistivity of about 0.5 ohm-cm nominal. Schottky junctions formed at the surface of such layers have a turn-on voltage of about 0.45V@30 nA/sq. micron, and low reverse saturation current.
Unfortunately, the prior art Schottky structures have several characteristics that limit their usefulness as voltage clamps for collector-base junctions in certain important integrated circuits. These problems are principally 1) a high series resistance within the diode--which causes the "clamped" voltage to rise as a function of current through the Schottky diode--and 2) relatively wide carrier concentration variations on the semiconductor side of the Schottky interface--resulting in clamping voltage variations (and hence switching time variations) from one Schottky transistor to the next. These problems are described in the cross-referenced U.S. Pat. No. 5,150,177, of Robinson et al., entitled Schottky Diode Structure With Localized Well. In summary, when a prior art Schottky diode that suffers from the above-mentioned problems is part of a transistor, the result is a slower, less-predictable transistor than is otherwise achievable.
As illustrated in FIG. 1, a cross-sectional view of a typical prior art Schottky transistor configuration, the transistor portion includes a highly-doped collector surface region 1, an epitaxial layer 3, an underlying buried collector layer 4, and a base region 5. A Schottky contact 2 at the surface of the epitaxial layer 3 comprises in conjunction with the epitaxial layer 3 a Schottky diode structure that serves to limit voltage across the collector-base junction and to divert minority carriers away from the base region 5. This N type epitaxial layer is doped as previously indicated, with donor concentration relatively uniform.
Although the epitaxial layer's donor concentration of 1-3.times.10.sup.16 /cc produces a Schottky diode with good junction characteristics, it also leads to a high series resistance within the diode. This high resistance results in a current-dependent voltage drop through the bulk of the diode which adds to the drop across the Schottky junction during current flow, thus causing the "clamped" voltage across the collector-base junction to drift upward, leading to a minority-carrier build-up within the base. In order to provide a Schottky diode with adequate shunting capacity at a fixed voltage, it is necessary to reduce the diode's bulk resistance--i.e., the resistance of the current pathway through the semiconductor--to the rest of the circuit. The usual ways to decrease the resistance of a pathway are: a) decrease the resistivity of the material making up the pathway, b) decrease the pathway length, c) increase the pathway cross-section. These methods are not directly practicable in the current state of the art. That is, it is not possible to decrease the resistivity of the epitaxial layer without degrading the Schottky junction and other circuit components. Moreover, because of the other configurational requirements of the circuit structure, it is generally not possible to reduce the length of the path through the epitaxial layer. Finally, any increase in the diode's cross-section is at the cost of increasing parasitic diode capacitance and decreasing the number of active structures that can be formed on a given substrate. Thus, within the prior art constraints, overcoming the problem of high diode resistance must be balanced against the problems associated with an increase in the structure's dimensions.
The other problem associated with prior art Schottky diode fabrication methods is the above-mentioned variation in donor concentration from one device to the next. For a number of reasons, donor concentrations in the epitaxial layer can vary by as much as .+-.30% from semiconductor "wafer" to wafer, or within a wafer. In prior-art structures these variations resulted from the way in which donor concentrations were achieved; that is, the growth of the epitaxial layer controlled the donor levels in the wafer. In fact, actual donor concentrations in the epitaxial layer are not known until after the deposition process is completed. This fluctuation translates into characteristic variability for a given fabrication lot; it directly affects the forward voltage drop and the reverse current at the metal-semiconductor interface. It also directly affects the series resistance through the bulk of the epitaxial layer. In effect, the forward voltage drop, the reverse current leakage rate, and the series resistance may all vary significantly from one diode to the next.
Recent innovations in integrated circuit bipolar NPN transistor structure fabrication methods have led to increasing localization and miniaturization of active areas. See, for example, co-pending application Ser. No. 07/942,977, of Joyce et al., filed Sep. 10, 1992, entitled Improved Lateral PNP Transistor In A BICMOS Process. These new techniques include implanting relatively slow-diffusing N type atoms in a P type substrate (so as to form a buried N.sup.+ collector layer for the bipolar structure), using a collector-fabrication sequence: mask, etch, and implant. Next, a "sub-emitter collector region" of relatively fast-diffusing N type atoms in an N.sup.++ concentration is implanted into a portion of the buried collector layer. The implant is provided by means of a sub-emitter collector mask, etch, and implant sequence. The epitaxial layer of semiconductor material, with an N.sup.- concentration of donors, is then deposited on the substrate. The donor concentration in this new epitaxial layer, typically about 1-3.times.10.sup.15 donors/cc, has a lower donor concentration than is present in earlier prior-art epitaxial layers. The reason for this reduction is to minimize parasitic collector/base capacitance in the bipolar structure and to localize sub-elements of the integrated circuit, both increasing concerns as integrated circuits continue to be miniaturized.
In the recent fabrication processes, thermal cycling causes the sub-emitter collector region to up-diffuse, or "retrograde diffuse" through the epitaxial layer in a well-defined manner. This retrograde diffusion thus establishes a donor concentration gradient through these localized areas of the epitaxial layer. Near the epitaxial layer surface the donor concentration is lowest; near the epitaxial layer/substrate interface the concentration is greatest. The resistivity of the sub-emitter collector region therefore decreases from that region's surface to the underlying substrate. The donor concentration, at least in this region, is no longer controlled by the growth of the epitaxial layer; instead, the extent of the implant is the dominant factor.
Continuing this new procedure, isolation regions of silicon dioxide are formed--using a conventional active-area mask, etch, and growth sequence--in order to provide surface isolation between active areas. In the final stages of this new fabrication process, contact deposition regions are formed through a contact definition mask, etch and deposition sequence, using a passivating silicon nitride layer as the mask. A first metal is then deposited in the contact regions. (One way to establish a Schottky clamping transistor, is to extend the metal deposited over the base region to cover the surface of the adjacent epitaxial layer.) A second metal layer is then deposited over the first metal layer. It should be noted that an interlayer dielectric may be formed on the first metal layer, prior to second metal layer deposition, so as to separate the two metal layers.
A disadvantage of the reduced donor concentration in the epitaxial layer of the described structure is that the relatively light doping is unsuitable for a Schottky diode of the type needed. A diode formed utilizing that substrate has a much higher turn-on voltage as well as a higher reverse current leakage than would be acceptable or useful. Also, as addressed earlier, the series resistance through the epitaxial bulk is excessive with this light doping. All of these characteristics in a Schottky transistor would make it a less-than-optimal collector-base shunt for limiting minority carriers in the base region. Furthermore, the use of the low donor-concentration epitaxial layer as part of the Schottky diode would still result in performance variability problems from one diode to the next because the deposition process remains the same; only the donor concentration is lower.
As indicated, these problems have been noted and addressed in U.S. Pat. No. 5,150,177 for a generic Schottky diode. That disclosure describes a method of retaining the advantages of a relatively lightly-doped epitaxial layer while increasing the donor concentration in the Schottky diode region. That donor concentration is achieved by forming a localized NWell below the metal-semiconductor interface of the diode at the same time that the NWell of a PMOS transistor is formed. This localized well, of a size similar to that of the PMOS NWell, provides a pathway with increasing donor concentration and hence provides a low-resistance pathway for the current, while continuing to allow a suitably high semiconductor resistance at the metal/semiconductor interface. In order to use this localized NWell technique in the fabrication of a Schottky transistor--i.e., to incorporate it into the fabrication of a bipolar junction transistor--it would have to be modified in order to permit the Schottky diode to be tailored to the dimensions of the transistor's active area. It would, for example, be unacceptable to simply incorporate a large localized Schottky diode well. This would increase the size of the transistor, and hence transistor capacitance, and decrease the device density possible on a given wafer.
Therefore, what is needed is an improved Schottky transistor formed using a Schottky diode as a collector-base junction clamp, where the Schottky diode has low reverse-leakage, a relatively low turn-on voltage and little characteristic variability from one structure to another. What is also needed is a Schottky transistor with a Schottky diode structure that has a relatively low-resistance current path through the bulk of the semiconductor material. Such a structure would increase the speed at which minority carriers are diverted away from the collector-base junction of the bipolar transistor and would minimize the required size of the transistor. Further, what is needed is a Schottky transistor with a Schottky diode structure that can be tailored to the specific current-diverting requirements of the particular bipolar transistor. Still further, what is needed is a new bipolar transistor fabrication process that includes the fabrication of the new clamping Schottky diode using existing bipolar mask sequences modified to include the formation of the new Schottky diode structure.